Francois Hollande convincingly won the French Socialist Party's presidential primary election to become the candidate who will try to unseat Nicolas Sarkozy next April and return a Socialist to the Elysee Palace for the first time in 17 years.

With about two-thirds of votes counted on Sunday evening, Hollande's victory in a runoff ballot against Martine Aubry was resounding, with his score topping 56 per cent.

Aubry has conceded saying: "I will invest all my strength and energy to ensure that he (Hollande) is the president of France seven months from now."

Hollande will be the chief challenger in next year's presidential elections to the conservative and unpopular Nicolas Sarkozy, who is widely expected to seek a second term.

Aubry and Hollande topped the first-round of the chief opposition party's primary held a week ago.

As results rolled in, Aubry said: "I warmly congratulate Francois Hollande, who is clearly ahead. His victory is unquestionable."

Aubry was the author of France's 35-hour workweek law and succeeded Hollande as Socialist Party leader.

Sunday's vote was France's first US-style open primary - any elector who said he or she supported the ideals of the left could vote.

Al Jazeera's Tim Friend, reporting from Paris, said socialists have not won an election in France since Francois Mitterrand in 1988 and so a win would be a political coup for leftist ideals.

Last Sunday, a bigger than expected 2.66 million voters turned out for the  first round, which was won by Hollande, a 57-year-old politician and former party leader, with only a narrow nine-point lead over Aubry.

'A good sign'

Voting in the southwestern town of Tulle, in his rural constituency, Hollande predicted an even bigger turnout, dubbing this a "good sign".

Hollande had won the backing of the four defeated first round candidates, and entered the run-off as favourite, but Aubry, 61, mounted a tough fight back, branding him a soft centrist without the steel to defeat Sarkozy.

Voting in Lille, where she is mayor, Aubry declared: "I urge all the French to vote for their convictions and with their hearts for real change."

Aubry, a former labour minister who gave France its 35-hour working week, has also attacked Hollande's lack of executive experience.

Hollande has tried to turn the attacks to his advantage, accusing Aubry of undermining party unity and suggesting that his lack of a track record makes it easier for him to run as a candidate of change.

Speaking to Al Jazeera, Stephane Rozes, a political strategist, said although the French presidential race is wide open because of the absence of ex-IMF chief Dominique Strauss-Kahn, who was brought down by sexual-assault allegations, it would be unwise for the socialists to assume the election, on May 6, 2012, was already in the bag.